Fiber optic backlighting panels are commonly used in flat panel liquid crystal displays and other selectively transmissive optical display devices, to display a high contrast image that is visible even under poor ambient light.
Typically, such prior backlighting devices utilize a single layer of optical fibers, each having a core surrounded by cladding, which are placed in intimate proximity to each other above an optically reflecting surface. Light is beamed into the optical fibers from a light source, such as a lamp. The light beamed in is propagated in the core of the optical conductor by means of partial internal refraction, and exits at one or more discontinuities in the fiber, providing local illumination in the region of the discontinuity. U.S. Pat. No. 4,845,596 to Mouissie discloses a manufacturing method for such a device whereby an outer sheath of the optical fiber is removed locally at various locations along the length of each fiber and a portion of the light beamed through the optical fibers exits therefrom at those points. This emerging light reflects off the reflective backing and illuminates a diffusing front layer. In accordance with the patent's teachings, the fibers preferably have a center-to-center spacing equal to at least twice the fiber diameter, in order to provide a relatively unobstructed path from the reflective backing to the surface being illuminated.
Backlighting panels have been commercially produced in which individual fibers in a closely packed parallel array provide a illumination pattern of regularly spaced points of illumination. In one known backlighting panel, the fibers are not completely terminated but are only provided with a hole or slot that has a lateral extent less than or equal to the fiber diameter and which is typically offset to some extent relative to the fiber axis as a result of production tolerances. Furthermore, in order to guarantee that the majority of fibers providing light to a downstream region will be free from any upstream discontinuities, the lateral spacing of the terminations is typically about 150% of the center-to-center spacing of the fibers As a result, many of the terminations are not complete and a significant portion of the light beamed into the fiber (perhaps 30%) does not exit at a designated termination point Moreover, even if a fiber is provided with a hole which extends across the entire fiber diameter, a significant portion (perhaps 20%) of the light exiting the hole from the upstream portion of fiber will travel in a forward direction across the hole until it reenters the downstream portion of the fiber on the other side of the termination. Accordingly, a backlighting panel manufactured in accordance with such a prior art process will provide a somewhat uniform distribution of light, but is relatively inefficient. In another commercially available backlighting panel, individual fibers in a closely packed parallel array are each provided with a single termination at which the light may exit, using a programmed laser to cut across the entire array in a series of oblique angles forming a zig-zag pattern. Such a technique makes more efficient use of the light, but the illumination is less uniform with relatively large dark areas between two adjacent diverging cuts. This lack of uniformity can be alleviated by means of a diffuser between the backlighting panel and the device being illuminated, but at the cost of additional bulk and expense.
A need thus exists for an improved fiber optic backlighting panel and a method for making such fiber optic panels, whereby a predetermined pattern of background illumination may be produced by a compact and efficient device that is easy to manufacture.